When it comes to the 7200's, you're in luck because life couldn't be easier when it comes to uninstalling/reinstalling HD's in these models.
The factory installed hard drives in these models are at the front of the case near the middle in the top drive bay (beside the floppy drive). I believe there is one screw holding the drive assembly in place, so just remove that and take the drive out. The 7200 case design also has one free drive bay (two free drive bays in the desktop G3's similar design), and it is already wired for power and SCSI, so all you need to do is put a sled on the hard drive, slide it into the drive bay and connect SCSI and power (of course, you can't have two SCSI devices on the same bus with the same ID number, so change the number using the dip switches on the hard drive. Apple mostly used Quantum drives in the 7200's, and the dip switches are located either on the circuit board under the drive or in-between the SCSI and power ports on the back).
As for the 9500, it's drive will work fine in any 7200 as well since they're all SCSI. I'm not sure which G3 model you have, so I can't tell you about that.
Let me know if you want anything clarified